Synthetic ester lubricants



United States Patent 3,360,465 SYNTHETIC ESTER LUBRICANTS Murray Warrnan, East Brunswick, N.J., assignor to Drew Chemical Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed May 22, 1964, Ser. N 369,600 2 Claims. (Cl. 252--56) This invention relates to synthetic lubricant compositions containing pentaerythritol mixed esters. More particularly, the invention relates to synthetic lubricant compositions which are especial-1y adapted for use with aircraft engines of the turbo-propeller and turbo-shaft types containing pentaerythritol mixed esters. It also relates to a method of lubricating such aircraft engines. This application is a containuation-in-part of co-pending application Ser. No. 287,219 filed June 12, 1963 and now abandoned.

The commercial and military demands for faster and higher-pressure aircraft jet engines have created stringent requirements for the lubrication of the engine to insure safe flight and prevent undue wear on the parts of the engine. Modern lubricants must be able to withstand bearing temperatures considerably above the 300F. limit for earlier turbine engines for many hours without undergoing chemical change. They must also be able to retain lubricity at a temperature of about 210 F. at a kinematic viscosity preferably in the range of about 4.9 to 5.5 centistokes and yet they must remain liquid with an initial kinematic viscosity of no more than 13,000 centistokes, and preferably lower than 10,000 centistokes, at temperatures down to --40 F.

While it is recognized that pentaerythritol esters generally perform satisfactorily as lubricants because they possess good thermal stability characteristics, they do not always meet the aforementioned viscosity requirement for jet engines at temperatures ranging from 40 F. to 210 F. At high temperatures, certain esters lose their cohesiveness indicated by an exceedingly low kinematic viscosity, and they tend to become thin. This tendency represents a loss of lubricity. At low temperatures, on the other hand, some esters tend to crystallize or form solids, or else they are too viscous to flow smoothly and freely. Esters having such characteristics could seriously impede the eflicient operation of the aircraft engine and even damage the moving parts of the engine.

One object of this invention is to provide lubricant compositions which possess satisfactory kinematic viscosities at temperatures from about 40 F. to 210 F.

Another object is to provide pentaerythritol esters which have initial kinematic viscosities below 10,000 centistokes at 40 F.

A further object is to provide novel lubricant compositions which satisfy the lubricity requirements of jet engine specifications.

The aforementioned and other objects I accomplish by providing novel lubricant compositions which are liquid and possess lubricity at temperatures between -40 F. and 210 F., containing as an essential ingredient an esterification product of pentaerythritol and a mixture of two to six monocarboxylic alkanoic acids having from 5 to 9 carbon atoms, in which mixture the average number of carbon atoms is in the range of 6.0 to 7.25 and the maximum number of carbon atoms contributed by the straight-chain 8- and 9- carbon acids is 60% and the maximum number contributed by branched chain acids is 45% of the average number of carbon atoms.

I have discovered that the novel pentaerythritol mixed esters of this invention have satisfactory initial kinematic viscosities at 210 F. and below 13,000 centistokes at -40 F. if the mixed acid reactants, of from 5 to 9 car- 3,360,465 Patented Dec. 26, 1967 bon atoms, are present in such molar quantities that the total number of carbon atoms contributed by acids in the mixture having straight-chain 8 and 9 carbon atoms is below 60% of the average number of carbon atoms and the total contributed by all branched acids is below 45%. In other words, upon complete esterification, the maximum number of carbon atoms contributed by the straight-chain 8- and 9-carbon acids is about 4.25 and that contributed by the branched acids is about 3.25.

In my preferred class of mixed esters, initial kinematic viscosities of less than 10,000 centistokes at -40 F. are obtained, using a technical or commercial grade of pentaerythritol, wherein the average number of carbon atoms of the acid mixture is in the range of 6.2 to 6.7, with percent carbon atom contributions of below 52% and 40% for the straight-chain 8- and 9-carbon acids and branched chain acids, respectively. Lubricant compositions containing these preferred esters have sufliciently low initial kinematic viscosities that they may be operative at temperatures even lower than 40 F.

The following sample calculation demonstrates the method of computing the average number of carbon atoms and the percent carbon atom contributions:

An acid mixture to be reacted with pentaerythritol contains mole percent of caprylic acid (denoted C 15 mole percent of pelargonic acid (C 45 mole percent of -a commercial isopentanoic acid (C containing 29.25 mole percent of normal valeric and 15.75 mole percent The sum of carbon atoms contributed by the methyl butyric and the 2-methyl pentanoic acids and the sum contributed by caprylic and pelargonic acids '(2.00+1.35=3.35) are each divided :by the average carbon atom content, 6.50, and multiplied by Carbon atom contribution: Percent Branched acids (1.6875/6.50) 100=25.93 Cg-I-Cg acids '(3.35/6.50) 100=51.5

The initial kinematic viscosity at 40 F. of an ester made from a technical grade of pentaerythritol and this acid mixture is 8425 centistokes; at 210 F., the initial kinematic viscosity is 5.10 centistokes.

In the preparation of the mixed esters of this invention, pentaerythritol is initially mixed with at least four moles per mole of pentaerythritol of a mixture of two to six monocarboxylic alkanoic acids of from 5 to 9 carbon atoms, selected according to desired molar proportions. A 5% to'1-0% excess of the acid mixture over the stoichiometric requirement is preferred. The reaction is carried out by heating the reaction mixture in a liquid phase under reflux conditions for a period of from 6 to 10 hours. The reaction temperature may be from about 275 to 450 F. The methylol groups of the pentaerythritol are esterified by the acids, causing four moles of water to be split off per mole of pentaerythritol. The esterification reaction is complete when the four moles of water are removed. For convenience in removing the water of reaction, the reaction may be carried out in the presence of an azeotroping agent, such as toluene, benzene or other suitable agent which in inert to the reactants.

The acids within the to 9 carbon atom range which may be used in my invention are the presently available commercial acids including the normal acids, valeric, caproic, caprylic and pelargonic, and the branched chain acids isopentanoic and 2-methyl pentanoic. The esters are made from either all straight-chain acids or mixed straight and branched chain acids. While I prefer to use straightchain caprylic and pelargonic acids, the lower acids, however, may be either all straight-chain acids or all branched or m-ixed. For example, the isopentanoic acid used herein is a typical commercial blend containing 60% to 65% by weight of n-valeric and 40% to 35% by weight of the Z-methyl and 3-methyl butyric acid isomers. For the purpose of this invention, the methyl butyric acid isomers are treated as identical acids in computing the percent of branched acid contribution, while the entire mixture, including normal valeric, has the same number of carbon atoms with regard to computing the average number of carbon atoms. Two to six different acids are used to prepare my lubricants and hence the reaction product may be either a mixed ester or a mixture of mixed esters. For convenience, I herein refer to the reaction product as simply a mixed ester or an ester-ification product.

The pentaerythritol which I use in describing my invention is the technical or commercial grade of pentaerythritol, such as Pente (a product of the Heyden Chemical Corporation), which usually contains about 88 to 90% monopent-aerythritol, to 1 2% dipentaerythritol, and less than 0.5% tripentaerythritol. -I also use a higher grade of pentaerythritol such as.Monopentek (also produced by the Heyden Chemical Corporation); this product contains about 98.5% monopentaerythritol and 1.5% dipentaerythritol. Owing to the lower amounts of polypentaerythritols in this latter product, a higher average number ofcarbon atoms and a greater percent of the straight-chain 8- and 9-carbon acids may be permitted. The carbon atom contribution of these .acids may be as high as 59.0% without exceeding a viscosity of 10,000 centistokes at -40 F. However, if the 60% limitation for these acids is exceeded the esters tend to crystallize, regardless of which pentaerythritol preparation is used. I conclude that for either type of pentaerythritol such a limitation is a particularly critical factor in the how characteristics of pentaerythritol mixed esters.

The following examples illustrate the invention. The parts stated are by weight. The percentages, unless denoted molar percentages, are also by weight.

7 EXAMPLE I Into a reaction vessel equipped with a thermometer, an agitator and a condenser were added 273.3 parts (2.0 moles) of pentaerythritol and a mixture consisting of 629.1 parts (6.16 moles) of a commercial isopentanoic acid (containing 65% by weight of n-valeric acid and 35% by weight of 2-methyl and 3-methyl butyric acids) and 417.7 parts (2.64 moles) of pelargonic acid. This acid mixture contains 24.5 mole percent of Z-methyl and 3-methyl butyric acid, 45.5 mole percent of normal valeric acid, and 30 mole percent of pelargonic acid. The average carbon atom content is 6.2, the percent C contribution is 43.5, and the percent branched acid contribution is 19.8.

The reaction vessel was closed and nitrogen gas was passed through at atmospheric pressure. The vessel was then heatedfrom about 300 to 450 F. for 7 to 9 hours under agitation until about 144 parts of water were condensed, indicating substantially complete esterification.

Excess acid was removed in vacuo, using a nitrogen gas blanket, at about 3 mm. Hg and a temperature of 392 F. The remaining liquid reaction mass was cooled to 158 F.', treated with dilute caustic solution followed by a water wash, slurried with 1% by weight of activated charcoal, dried at 230 F. and 3 mm. Hg for about 1 hour, and filtered to yield a clear, straw-colored liquid product.

Upon analysis, the product was determined to have the following characteristics:

'Specific gravity (ASTM D-1298) at 20/ 20 C. 0.995 to 1.000

Color (ASTM1500) 1.0 Neutralization number (ASTM D- 664) max 0.15 Hydroxyl value max 10.0 Saponification number 410110 Water content, percent (with Karl Fischer reagent) 0.05

Samples of the product were tested for specific properties with the following results:

Kinematic viscosity, centistokes (ASTM D The above properties show that this ester meets the requirements of present jet-engine military specifications, including Government specification Navy XWS-2994 (PP).

A lubricant composition containing essentially the ester of Example I was subjected to a l'50-hour test run in a Pratt and Whitney JT-3D test engine following the procedure of Pratt and Whitney Type II Lubricating Oil Specification for Aircraft Turbine Engines. This specification calls for adding the sample lubricant to the engine and running the engine through a series of 5-hour and 10-hour cycles, operating at 20,000 pounds of thrust at a temperature in excess of 600 F. The lubricant is periodically tested for thermal and oxidative stability.

\When the test was complete, the lubricant was found to comply with acceptable standards. It showed satisfactory thermal and oxidative stability, and the bearings and other metal parts in contact with the lubricant were clean and free of corrosion, wear or cracking.

EXAMPLE H Using the same procedure and apparatus as in Example I, 272.3 parts (2 moles) of pentaerythritol (using technical grade pentaerythritol) were mixed with an acid mixture consisting of 584.2 parts (5.72 moles) of commercial isopentanoic acid (containing 65 of n-valeric acid and 35% by weight of Z-methyl and 3-methyl butyric acid) and 487.4 parts (3.08 moles) of pelargonic acid. This mixture thus consists of 22.75 mole percent of Z-methyl and 3-methyl butyric acids, 42.25 mole percent of normal valeric, and 35 mole percent of pelargonic. The average carbon atom content is 6.4, the percent C contribution is 49.3 and the percent branched acid contribution is 17.8. The reactants were added to the reaction vessel and heated. When a total of 144 parts of water was collected in the condensate receiver, the heat was removed.

The clear liquid reaction product was refined as described above. The viscosities of this product were measured at the extreme temperatures required in the military specifications with the following results:

At Centistokes EXAMPLE III butyric acids, and 80 mole percent of an acid mixture having acids in approximately the following molar percent ranges:

In this acid mixture, the average carbon atom content is 6.56, the percent C and C contribution is 24.8, and the percent branched acid contribution is 5.3. The initial kinematic viscosities of the refined mixed esters were:

At- Centistokes 210 F. 5.02 '40 F. 7028 EXAMPLE IV Using the same procedure and apparatus as in Example I, other esters were prepared from pentaerythritol and acids at various molar amounts. Table I lists esters made with 98.5% monopentaerythritol and 1.5% dipentaery-thritol and Table II lists esters made with technical grade pentaerythritol. The tables include the molar percentages of the acids and the carbon atoms contributed by each (C.A.), the average number of carbon atoms in the acid mixture (being the sum of the carbon atoms contributed), the carbon atom percentages for the C and C acids and for the branched acids, and the initial kinematic viscosities of the ester at 210 F. and -40 F.

The accompanying tables indicate that the average carbon atom content of the esterifying acid mixture by itself is an inadequate means of predicting either the high or the low temperature viscosity. To illustrate, in Example 9, the average number of carbon atoms is 6.7 and in Example 10 it is 6.5. Surprisingly, however, the low temperature viscosity for Example 10 is higher than that of Example 9.

they are unsuitable at temperatures in the region of F. and below. For example, an ester was prepared from pentaerythritol and the following mixture of acids: caprylic, 35 mole percent; pelargonic, 15 mole percent; and n-valeric, mole percent. The average number of carbon atoms is 6.65, but the percent C and C contribution is 62.4. At 40 F., the ester crystallized. A second pentaerythritol ester was prepared from the following acid mixture: pelargonic 30 mole percent and Z-methyl pentanoic mole percent. The average number of carbon atoms is 6.9, but the percent branched acid contribution is 56.5. At 40 F., the initial kinematic viscosity of this ester was 14,530 centistokes. Many of these esters which alone are not suitable because they are too viscous or because they form solids at -40 F., may be blended with the preferred esters of this invention to provide blends of mixed esters which are satisfactory as aircraft lubricants, so long as the overall acid mixture conforms to my limitations.

In view of the known characteristics of other synthetic lubricating oils, I unexpectedly find that the novel class of mixed esters of this invention fulfill the viscosity requirements of present jet engine specifications, having initial kinematic viscosities at 40 F. of less than 13,000 centistokes while they retain sufiicient lubricity at 210 F. My preferred esters, having initial 40 F. viscosities of less than 10,000 centistokes, may be used successfully at even lower temperatures.

The esters of this invention are compatible with the usual lubrication additives which may be added to enhance the properties of the esters. Minor amounts of such lubrication additives as antioxidants, stabilizers, corrosion inhibitors, detergents and the like may be added, depending upon the additional properties sought. These additives generally increase the kinematic viscosities of the esters, and therefore selection of the ester should be made with regard to such physical effect. My esters also have both excellent storage stability and chemical stability during use.

TABLE I Caprylic (Ca Pelargonic Ca) n-Valeric (C Z-Methyl Pentanoic (Ca) Average 05+ C5 Branched Viscosity Example No. No. of Contribu- Acid Conat 210 F./

Carbon tion trlbution 40 F. M01. C.A. Mol. (1A. M01. C.A. Mol. C.A. Atoms (percent) (percent) (centipercent percent percent percent stokes) TABLE II Pelargonic n-Valerlc Methylbutyric Z-Methyl- Caprylie (Ca) (Ca) (C (C Pentanoic Average Ca-I-Cg Branched Viscosity (06) N0. of Contribu- Acid Conat 210 F./ Example N0. Carbon tion tnbution -40 F.

Atoms (percent) (percent) (centi- Mol. C.A. M01. (LA. Mol. (LA: Mol. C;A. M01. C.A. stokes) percent percent percent percent percent I have found that the conditions herein described must be complied with in the selection of the acids for preparing mixed pentaerythritol esters, otherwise the initial kinematic viscosity of the esters at '40 F. may exceed 13,000 centistokes and some esters may even commence to crystallize. In either event, while the ester does not lose The foregoing description and all reasonable modifica- 70 tions thereof which will be obvious to those skilled in the art are considered within the scope of my invention, as defined by the following:

What I claim is:

1. A lubricant composition adapted for use at 40 F.

all of its desired lubricity at moderately low temperatures, 75 consisting essentially of an ester of pentaerythritol and a mixture ofalkanoic acids consisting essentially of about 25 mol percent caprylic acid, about mol percent methyl pentanoic acid, in the range of from about 15 to about pelargonic acid, and in the range of from about 40 to about 45 mol percent valeric acid, said mixture' of alkanoic acids having in the range of from about 6.5 to about 6.7 carbon atoms per acid molecule, said pelargonic and caprylic acids consisting of from about 51 /2 to about 57 mol percent of said alkanoic acids, said alkanoic acids consisting of not more than 15 mol per cent of branched acids, said composition being characterized by remaining liquid and also maintaining its lubricity at -40 F, and further characterized by an initial kinematic viscosity of not more than about 6800 centistokes at 40 F.

2. A lubricant composition adapted for use at 40 F. consisting essentially of a pentaerythritol ester of a mixture of 1alkanoic acids consisting essentially in the range of from about to about mol percent pelargonic acid, in the range of from about 39, to about 45 /z mol percent n-valeric acid, and in the range of from about 22% to about 28 percent of a mixture of methyl butyric 8 acid isomers, said composition being characterized by remaining liquid and also maintaining its lubricity at F., and an initial kinematic viscosity of not more than 8425 centistokes at 40 F.

References Cited UNITED STATES PATENTS OTHER REFERENCES Barnes et al., Synthetic Ester Lubricants," Lubrication Engineering, August 1957, pages 454-458.

DANIEL E. WYMAN, Primary Examiner.

PATRICK P. GARVIN, JOSEPH R. LIBERMAN,

Examiners. 

1. A LUBRICANT COMPOSITION ADAPTED FOR USE AT -40*F. CONSISTING ESSENTIALLY OF AN ESTER OF PENTAERYTHRITOL AND A MIXTURE OF ALKANOIC ACIDS CONSISTING ESSENTIALLY OF ABOUT 25 MOL PERCENT CAPRYLIC ACID, ABOUT 15 MOL PERCENT METHYL PENTANOIC ACID, IN THE RANGE OF FROM ABOUT 15 TO ABOUT 20% PELARGONIC ACID, AND IN THE RANGE OF FROM ABOUT 40 TO ABOUT 45 MOL PERCENT VALERIC ACID, SAID MIXTURE OF ALKANOIC ACIDS HAVING THE RANGE OF FROM ABOUT 6.5 TO ABOUT 6.7 CARBON ATOMS PER ACID MOLECULE, SAID PELARGONIC AND CAPRYLIC ACIDS CONSISTING OF FROM ABOUT 51 1/2 TO ABOUT 57 MOL PERCENT OF SAID ALKANOIC ACIDS, SAID ALKANOIC ACIDS CONSISTING OF NOT MORE THAN 15 MOL PERCENT OF BRANCHED ACIDS, SAID COMPOSITION BEING CHARACTERIZED BY REMAINING LIQUID AND ALSO MAINTAINING ITS LUBRICITY AT -40*F, AND FURTHER CHARACTERIZED BY AN INITIAL KINEMATIC VISCOSITY OF NOT MORE THAN ABOUT 6800 CENTISTOKES AT -40*F. 